1. Field of the Invention
The present invention relates to a radio frequency (RF receiver for a code division multiple access (CDMA) mkobile communication base transceiver station (BTS) system and, ore particularly, to an RF receiver for a CDMA mobile communication BTS system contrived to down-convert RF signals of 3 frequency allocation (FA)'s to intermediate frequency (IF) signals, convert the 3FA IF signals to digital signals, and digitally perform the FA-based quadrature phase shift keying (OPSK) demodulation and channel filtering.
2. Description of the Related Art
A general CDMA mobile communication BTS system is composed of a switching system and cell equipment. Here, the system is constituted by a plurality of functional devices which are implemented with various forms of equipment.
The nucleus of the mobile communication BTS system is implemented with a digital self embedding a channel card, a sector interface card, an analog common card and a terminal card, and a transceiver self for up-converting an IF signal output from the digital self to an RF signal, or down-converting the RF signal to the IF signal.
The transceiver self also embeds a sector interface card for combining the forward baseband signals received from the channel card and up-converting the combined signals to IF signals. The sector access card combines the baseband transmission signals received from the analog common card and amplifies the combined signals. The combined signals are passed through a low-pass filter (LPF) into IF signals, i.e., the signals are combined with 0° or 90° delay signals of 4.95 MHz and sent to an RF rack as IF signals of 4.95 MHz via a band-pass filter (BPF). The RF rack converts the IF signals of 4.95 MHz to RF signals.
Now, a description will be given to a general CDMA digital mobile communication BTS system with reference to FIG. 1.
FIG. 1 is a schematic block diagram of the general CDMA digital mobile communication BTS system.
As illustrated, the CDMA digital mobile communication BTS system includes: a BTS control processor (BCP) 2 for entirely managing and controlling a BTS; a BTS interconnection network (BIN) 3 for performing the function of a packet router between the BTS and a base station controller (BSC) 1 via a line E1 or T1, and interfacing high-level data link control (HDLC) packet data between the respective processors provided in the BTS; a time and frequency unit (TFU) 4 for generating a reference frequency and a timing sync signal to acquire synchronization between the respective processors in the BTS and timing synchronization with neighboring BTS's; a digital unit (DU) 5 for modulating/demodulating data and voice signals communicated via CDMA channels; and an RF unit (RFU) 6 for converting an RF signal received from a mobile station to an IF signal, transmitting the IF signal to the DU 5, converting the IF signal received from the DU 5 to the RF signal and amplifying the RF signal to a predetermined level for spatial distribution. Here, the RFU 6 is divided into an RF transmitter for converting the IF signal to the RF signal and transmitting the RF signal to the mobile station via an antenna, and an RF receiver for converting the RF signal received from the mobile station to the IF signal.
The RF receiver according to prior art will now be described in further detail with reference to FIG. 2.
FIG. 2 is a schematic block diagram of the RF receiver for the CDMA mobile communication BTS system according to prior art, which includes two antennas 10 and 15 to attain diversity, FA-based RF down-converters 30, 31 and 32, FA-based analog IF units 40, 41 and 42, and FA-based channel cards 50, 51 and 52.
The RF receiver supporting antenna diversity has two physical reception paths of “0” and “1”. The first antenna 10 and a first receive block 20 are assigned to the reception path of “0”, the second antenna 15 and a second receive block 25 assigned to the reception path of “1”.
All the first, second and third RF down-converters 30, 31 and 32 and the first, second and third analog IF units 40, 41 and 42 have two blocks for independently processing the reception paths of “0” and “1” and support both the reception paths of “0” and “1”.
The first and second antennas 10 and 15 and the first and second receive blocks 20 and 25 are used in common to all FA's assigned, and the first to third RF down-converters 30, 31 and 32 and the first to third analog IF units 40, 41 and 42 are used by FA's.
The first, second and third channel cards 50, 51 and 52 are provided at least one in number and used by FA's. For example, the RF receiver for the CDMF system supporting 4 FA's has two antennas, two receive blocks, four RF down-converters, and four analog IF units, and thus includes at least four channel cards.
Now, a description will be made as to an operation of the above-structured RF receiver supporting 3 FA's according to prior art.
First, the first and second receive blocks 20 and 25 receive RF signals from the first and second antennas 10 and 15, respectively, limiting the band of the signal using a band-pass filter (not shown), and amplify the band-pass filtered signals to a predetermined level with a linear noise amplifier (not shown). The first and second receive blocks 20 and 25 then output the amplified RF signals, i.e., the RF signals on the reception paths of “0” and “1”, respectively, to the first, second and third RF down-converters 30, 31 and 32.
The first, second and third down-converters 30, 31 and 32 receive the RF signals on the reception paths of “0” and “1” from the first and second receive blocks 20 and 25, down-convert the received RF signals to IF signals with a two-stage mixer (not shown) and a local oscillator (not shown) and output the converted IF signals to the FA-based first, second and third analog IF units 40, 41 and 42.
That is, each of the first, second and third RF down-converters 30, 31 and 32 first down-converts the RF signals to IF signals of about 70 MHz via the local oscillator and the mixer provided at the first stage and second down-converts the IF signals of the 70 MHz to IF signals of the 4.95 MHz via the local oscillator and the mixer provided at the second stage, concurrently limiting the band of the signals with an SAW filter having a passband of 1.25 MHz that corresponds to the bandwidth of one FA.
Each of the first, second and third analog IF units 40, 41 and 42 by FA's receives the IF signals of corresponding FA's on the reception paths of “0” and “1” output from the FA-based first, second and third RF down-converters 30, 31 and 32, divides the IF signals into I (In-phase) and Q (Quadrature) channels, down-converts the I/Q channel IF signals to a baseband to perform quadrature phase shifting keying demodulation, and A/D converts the I/Q channel analog baseband signals to digital baseband signals.
The FA-based analog IF units 40, 41 and 42 multiplex the I/Q channel digital baseband signals on the reception paths of “0” and “1” and transmit them to the channels cards 50, 51 and 52 corresponding to the respective FA's.
The FA-based channel cards 50, 51 and 52 receive the multiplexed I/Q channel digital baseband signals on the reception paths of “0” and “1” to perform CDMA demodulation by FA's.
As the conventional RF receiver for the CDMA mobile communication BTS system uses RF down-converters and analog IF units by FA's, the system can by expanded by no more than one FA during FA expansion.
Therefore, in order to process multiple FA's, for example, 3 FA's, there are needed to provide three RF down-converters and three analog IF units with a consequence of increased size of the RF receiver and hence the BTS system, thus raising the hardware cost.
Furthermore, the RF receiver has a limitation on reducing the size of the board due to a need of two mixers in the RF down-converter.